1. Field of the Invention
The present invention relates to an active matrix display device wherein the drive of a thin film luminescent element such as an electroluminescent element (hereinafter referred to as an xe2x80x9cEL elementxe2x80x9d) or a light emitting diode element (hereinafter referred to as an xe2x80x9cLED elementxe2x80x9d), which emits light when drive current passes through a luminescent thin film such as an organic semiconductor film, is controlled by a thin film transistor (hereinafter referred to as a xe2x80x9cTFTxe2x80x9d).
2. Description of the Related Art
An active matrix display device has been proposed which employs a current-controlled luminescent element such as an EL element or an LED element. All these luminescent elements are self-luminescent, making them advantageous in that they do not need a backlight that is required in the case of a liquid crystal display device and that they depend less on viewing angles.
FIG. 4 is a block diagram of an active matrix display device employing an EL element that emits light by means of a charge-injection type organic semiconductor thin film. Disposed on a transparent substrate 10 of an active matrix display device 1A are a plurality of scanning lines gate, a plurality of data lines sig extendedly provided in such a direction that they intersect with the direction in which the scanning lines gate are extendedly provided, a plurality of common feeder lines com parallel to the data lines sig, and pixels 7 formed in a matrix by the data lines sig and the scanning lines gate. A data side drive circuit 3 and a scanning side drive circuit 4 are configured for the data lines sig and the scanning lines gate. Provided for each pixel 7 are a conduction control circuit 50 to which scanning signals are supplied via the scanning lines gate, and a thin film luminescent element 40 that emits light in accordance with image signals supplied from the data lines sig via the conduction control circuit 50. The conduction control circuit 50 is constituted by a first TFT 20 in which scanning signals are supplied to a gate electrode thereof via the scanning lines gate, a retention capacitor cap that retains image signals supplied from the data lines sig via the first TFT 20, and a second TFT 30 in which the image signals retained by the retention capacitor cap are supplied to a gate electrode thereof. The second TFT 30 and the thin film luminescent element 40 are connected in series between an opposing electrode op and the common feeder lines com to be discussed hereinafter. When the second TFT 30 is placed in an ON state, drive current passes through the common feeder lines com, causing the thin film luminescent element 40 to emit light, and the luminescent state is retained by the retention capacitor cap for a predetermined period of time.
FIG. 5 is a top plan view showing one of the pixels included in the active matrix display device shown in FIG. 4. FIGS. 6(A), (B), and (C) are a sectional view taken at the line A-Axe2x80x2, a sectional view taken at the line B-Bxe2x80x2, and a sectional view taken at the line C-Cxe2x80x2 of FIG. 5, respectively.
In the active matrix display device 1A having such a configuration, the first TFT 20 and the second TFT 30 are formed in the same process by utilizing island-like semiconductor films in every pixel 7 as shown in FIG. 5 and FIGS. 6(A) and (B). The first TFT 20 has a gate electrode 21 configured as a part of the scanning line gate. In the first TFT 20, the data line sig is electrically connected via a contact hole of a first interlayer insulating film 51 to one end of a source and drain region, while a drain electrode 22 is electrically connected to the other end thereof. The drain electrode 22 is extendedly provided toward the region where the second TFT 30 is formed. A gate electrode 31 of the second TFT 30 is electrically connected to the extendedly provided portion via a contact hole of the first interlayer insulating film 51. A relay electrode 35 is electrically connected to one end of the source and drain region of the second TFT 30 via the contact hole of the first interlayer insulating film 51. A pixel electrode 41 of the thin film luminescent element 40 is electrically connected to the relay electrode 35 via a contact hole of a second interlayer insulating film 52.
As can be seen from FIG. 5 and FIGS. 6(B) and (C), the pixel electrode 41 is formed independently for each pixel 7. On the upper layer side of the pixel electrode 41, an organic semiconductor film 43 and the opposing electrode op are laminated in this order. The opposing electrode op is formed so that it covers at least a display section 11.
Referring back to FIG. 5 and FIG. 6(A), the common feeder line com is electrically connected to the other end of the source and drain region of the second TFT 30 via the contact hole of the first interlayer insulating film 51. An extendedly provided portion 39 of the common feeder line com opposes an extendedly provided portion 36 of the gate electrode 31 of the second TFT 30, with the first interlayer insulating film 51 sandwiched therebetween as a dielectric film thereby to form the retention capacitor cap.
The active matrix display device 1A provides a great advantage in that the opposing electrode op deposited on the transparent substrate 10 obviates the need for laminating an opposing substrate, differentiating itself from an active matrix liquid crystal display device. However, the thin film luminescent element 40 is simply covered by the thin opposing electrode op, so that moisture or oxygen intrudes into the organic semiconductor film 43 by diffusing and transmitting through the opposing electrode op, leading to a danger of deteriorated luminous efficiency, a higher drive voltage (shift of a threshold voltage to a higher voltage side), and deteriorated reliability of the thin film luminescent element 40. To prevent the entry of the moisture or oxygen, the conventional active matrix display device 1A has been employing a method wherein at least the display section 11 is covered by an opposing substrate, and the outer periphery of the opposing substrate has been sealed. This method, however, inevitably sacrifices the advantage over the liquid crystal display device.
Accordingly, an object of the present invention is to provide an active matrix display device capable of protecting a thin film luminescent element from moisture, etc. by means of a simple structure.
The active matrix display device in accordance with the present invention has the following configuration.
The active matrix display device has a display section on a substrate, the display section being formed by a plurality of scanning lines, a plurality of data lines intersecting the scanning lines, and a plurality of pixels formed in a matrix by the data lines and the scanning lines, each of the pixels having a conduction control circuit including a thin film transistor to which a scanning signal is supplied to a gate electrode thereof via the scanning lines, a pixel electrode formed for each pixel, a luminescent thin film deposited on an upper layer side of the pixel electrode, and a thin film luminescent element equipped with an opposing electrode which is formed at least on an entire surface of the display section on an upper layer side of the luminescent thin film, and the thin film luminescent element emitting light in accordance with image signals supplied from the data lines via the conduction control circuit, wherein: a protective film is formed on the upper layer side of the opposing electrode, which covers at least a region where the opposing electrode is formed.
According to the configuration, the thin film luminescent element can be protected against moisture, etc., that is diffused or transmitted through the opposing electrode since the protective film is formed on the upper layer side of the opposing electrode of the thin film luminescent element. Hence, it is possible to prevent deteriorated luminous efficiency, a rise in the drive voltage (the shift of a threshold voltage to the higher voltage side), deteriorated reliability, etc. in the thin film luminescent element. Moreover, the protective film can be easily formed by using a semiconductor process, so that it does not add to the manufacturing cost of the active matrix display device. Thus, the reliability of the active matrix display device can be improved, while retaining the advantage of the active matrix display device employing the thin film luminescent element in which no opposing substrate is required to be deposited. Furthermore, since the protective film protects the thin film luminescent element, the material used for the opposing electrode may be selected from the viewpoint mainly of the luminous efficiency or the drive voltage of the thin film luminescent element, thus providing another advantage in that the material is not limited to one having high performance to protect the thin film luminescent element.
In the present invention, it is preferable that the luminescent thin film is partitioned by an insulating film formed on a lower layer side of the opposing electrode so that it is thicker than the organic semiconductor film. In the active matrix display device employing the thin film luminescent element, the opposing electrode is formed at least over the entire surface of the display section and opposes the data line; therefore, a large parasitic capacitor is produced on the data line as is. According to the present invention, however, the presence of the thick insulating film between the data line and the opposing electrode makes it possible to inhibit the parasitic capacitor from being produced on the data line. As a result, the load on a data side drive circuit can be reduced, enabling reduced power consumption or quicker display operation. In addition, the insulating film formed as mentioned above can be used as a bank layer for preventing a discharge liquid from spilling out when forming a luminescent thin film in a region partitioned by the insulating film by the ink-jet process.
In the present invention, preferably, the opposing electrode is formed of, for example, an aluminum film containing an alkali metal. When the opposing electrode is formed of such a film, the possibility of moisture, etc. being diffused or transmitted is higher; hence, the effect of the formation of the protective film is remarkable.
In the present invention, the protective film may be formed of an insulating film such as a silicon nitride film, or it may be formed of a conductive film of a metal having a high melting point or an alloy thereof. Further, alternatively, the protective film may be formed of a conductive film such as a pure aluminum film, an aluminum film containing silicon, or an aluminum film containing copper. Further, the protective film may be formed of two layers consisting of a conductive film and an insulating film. When the protective film deposited on the opposing electrode is formed of a conductive film, the same effect that can be obtained from lowering the electrical resistance of the opposing electrode can be achieved. When the thick insulating film is formed partitioning the region where the organic semiconductor film is formed, the large difference in level produced by the insulating film may cause disconnection of the opposing electrode formed on the upper layer side thereof. Forming the protective film deposited on the opposing electrode of a conductive film makes it possible to prevent the disconnection of the opposing electrode because the conductive film forms a redundant wiring structure. Accordingly, even when the thick insulating film is formed around the organic semiconductor film to suppress a parasitic capacitance in an active matrix display device, the disconnection of the opposing electrode formed on the upper layer of the insulating film does not occur, enabling improved display quality and reliability of the active matrix display device to be achieved.
In the present invention, the conduction control circuit is preferably provided with the first TFT wherein the scanning signals are supplied to the gate electrode thereof, and the second TFT wherein the gate electrode thereof is connected to the data lines via the first TFT, and the second TFT and the thin film luminescent element are connected in series between the common feeder line for supplying drive current, which is configured separately from the data lines and the scanning lines, and the opposing electrode. In other words, the conduction control circuit could be constructed by one TFT and a retention capacitor; however, it is preferable to configure the conduction control circuit of each pixel by two TFTs and a retention capacitor to accomplish higher display quality.